Translation:You eat fish.
The most usual way is to resort to using demonstrative adjectives e.g. 你吃那条鱼.
Be aware that I say it is generic not just because it has nothing before it, but also it is in an isolated sentence. 你吃鱼，我吃鸡 could be specific (as when there are before us a Fish-O-Filet and a McChicken). 你把鱼吃了 (literally You render the fish eaten) is also specific (as when I see you eating the fish I bought to be saved for later.)
Duo, claim the fee from the burger shop.